the castor-oil plant (also used for other plants)
萆 centers on a specific plant: the castor-oil plant, and by extension can refer to other similar plants.
萆 combines the grass radical 艹 with 卑, which likely contributes a phonetic element. The character was used to name the castor-oil plant and other plants.
The grass radical 艹 sits above 卑, like a lowly weed. Picture a humble castor-oil plant growing close to the ground.
For ヒ, imagine the castor-oil plant growing on a hill: hill -> ヒ, and the plant thrives on the sunny slope.
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